In developing the sensor element for a pressure sensor a very large number of technical parameters exist which must be considered. These parameters are of a thermal, electrical, mechanical or chemical nature. Each such group contains itself a multitude of technical parameters and further various properties interact between the groups as applied to a sensor element. Chemical characteristics influence surface coatings, which influence thermal and mechanical properties. The density and impermeability influence possible structural thicknesses what influences the mechanical characteristics, etc. A search ol the characteristics of various conceivable materials concludes usually that the following materials can possibly be used: metals of special types such as the alloys "Inconel" and "Hastalloy", semi-conductor materials, for example silicon, ceramic materials, for example aluminum oxide.
A very large work has been made in developing different types of sensor elements. In order to use the sensor element it must, however, be mounted in a housing, casing or similar device, to obtain a complete or finished pressure sensor, which is ready to be used after a simple connection to some volume, where a medium is present. However, the mounting or attachment method for forming a complete pressure sensor can often result in the fact that the high precision of the very sensor element is lost. It depends on the fact that, again, in the mounting process problems in regard of the materials used appear which are for example of the following kind. For metals, their high coefficients of thermal expansion result in thermal stresses, they are not impermeable to some gases, they are deformable. Semiconductor materials are sensitive to temperature and are easily attacked chemically, what can have the consequence that a system comprising oil capillaries must be used to conduct the pressure that is to be measured to a pressure sensitive surface. The attachment or securing of parts of ceramic materials is often performed, due to the difficulty of working these materials and their production at very high temperatures, must often by means of O-rings, which do not possess a sufficient chemical resistance or inertness.
The housing of the pressure sensor is nearly always made of metal and thus, to this metal the very sensor element must be attached or clamped. A suitable ceramic material for use in the construction of sensor elements is glass ceramics, since components or parts of this material can be produced and joined/bonded at temperatures which are rather low in this context. If the sensor element thus is to be bonded directly to metal, it is a critical problem to adapt the coefficient of thermal expansion of the metal to which the sensor element is to be mounted or clamped, further of the material in the very sensor element and also of the bonding or joining material used.
Sensor elements for pressure sensors based on ceramic materials and constructed as dilatation sensors or capacitive sensors can comprise various ceramic materials. Then, often ceramics based on aluminum oxide is used but also glass ceramics is used. In the production of sensor elements based on aluminum oxide the various surface coating processes and the procedures for burning/fusioning the various elements included in a sensor element to each other must be performed at a significantly higher temperature than for glass ceramics. To design, for sensor elements based on aluminum oxide, a sequential order for all these processes comprising surface coating procedures of various kinds and burning/fusion processes, which does not destroy the result of earlier coating processes and other processes in the sequence of procedural steps in the production, must be considered very difficult. Further, to adapt for such sensor elements physical parameters, for example coefficients of thermal linear expansion, over a wide temperature range is also very difficult. Therefrom it is obtained as a natural consequence that the high temperatures used imply that mechanical stresses will exist in a finished sensor element, what has naturally various resulting, difficult effects when using the sensor element in a pressure sensor. A rigid securing or attachment of a sensor element constructed of ceramic materials to a metal part, for example a ring of stainless steel, can thus in most cases not be performed owing to the built-in mechanical stresses obtained from the thermotechnical conditions during the production process of the sensor element.
A prior alternative is to clamp a sensor element, which as conventional has the shape of e.g. a flat round plate or chip, to an O-ring, so that it is pressed against one of the flat surfaces of the sensor element at a region adjacent to the edge of this surface. The clamping force can be produced by a threaded ring acting on the opposite side of the sensor element and at a region adjacent the edge of this surface. Another previously known alternative is that a force from a fluid, the pressure of which is to be measured, is transferred from a primary measuring diaphragm through an auxiliary fluid to a surface of the sensor element. The clamping of the sensor element can in the latter case be accomplished in a simpler way, owing to the fact that the characteristics of the auxiliary fluid, e.g. silicon oil, which is used for transferring the pressure to be measured, are known. Such methods can however only achieve limited performance due to mechanical elasticity and mechanical instabilities in an O-ring or a silicon oil, respectively, so that a large accuracy and a rapid sensor response cannot most often be obtained.
A rigid securing/mounting of the sensor element is according to the discussion above necessary in order to achieve precision sensors for measurements in for instance industrial areas, where low pressures are used, such as the semi-conductor industry, but also for pressure sensors intended for measurement of ordinary pressures a rigid attachment/mounting of the sensor element produces distinctly superior characteristics.
In a prior sensor, see the U.S. Pat. No. 5,249,469, which can be made having different dimensions for measuring pressures of different magnitudes, both vacuum pressures and atmospherical pressures, the sensor element is almost completely arranged within a volume filled with a fluid the pressure of which is to be measured. The sensor element is attached to a sensor house by means of two slender tubes having a very small cross-sectional area of the material of the tubes. In the use of the pressure sensor for measurement of low pressures, for measurement of a vacuum, the whole measurement element is thermotechnically isolated from the surroundings and is influenced little by changes in the ambient temperature. The heat transport to and from the sensor by means of convection and radiation is small and the heat transport occurs substantially as conduction of heat through the slender tubes.
At rapid temperature changes, in e.g. the use at a low pressure, in a rapid pumping down to a vacuum, in a rapid inlet of fluid, the gas, e.g. air, the pressure of which is to be measured, will expand or contract. It results in a small cooling or heating respectively of the gas within the measurement volume, with which the sensor element is in contact, and thence also of the sensor element itself.
This phenomenon is in particular embarrassing for a rapid pumping to a low pressure, since the sensor element then, comprising a mount according to the discussion above comprising slender tubes, is thermotechnically well isolated both from the housing to which it is attached and which in many cases rather rapidly will adopt the new temperature due to the fact that it comprises large surfaces and is made of metal, and from the gas itself. The temperature equalization between the sensor element and the surroundings occurs in this case very slowly through the slender tubes and the zero position of the sensor is then displaced or offset during a rather long time. The return process to the state existing before such a change can in the worst cases comprise a time of up to the magnitude of order of hours, which naturally cannot be accepted. In FIGS. 1a-1c time diagrams are illustrated showing the zero position of the output signal for different heat conduction cases in relation to the surroundings. In the normal case or in the case comprising a good heat conduction according to FIG. 1a the zero level thus returns to its normal value after a limited time period. In other cases for which a good thermotechnical isolation is provided between the sensor element and the material in the surroundings, the zero level dependence of time can look as is illustrated by the curves of FIGS. 1b and 1c, comprising the long period mentioned above for a return to the zero level, in FIG. 1b comprising a positive zero point offset and in FIG. 1c a negative zero point offset during a rather long time. The different curve shapes in FIGS. 1b and 1c depend primarily on the processes used in the production of the sensor element itself, i.e. the interior characteristics thereof. It is possible, in principle, to compensate for the rapid changes of the zero level in an electronic way but practically it is combined with large difficulties, since the sensor element itself is in this construction not easily available for arranging temperature sensors such as thermistors.
A sensor element suspended in two slender tubes is generally not easily produced. However, it can be used where a high cost of the pressure sensor can be accepted and for small volumes, for instance in the production of semi-conductor elements. For other industrial sensors where large numbers of sensors are required but not as extreme performance, this type of attachment is impossible. Industrial sensors should also preferably be able to be operated for measurement of pressures in different media, both gases and liquids. For liquids the mounting by means of two slender tubes does not work.
Another complication in an attachment by means of slender tubes is the welding thereof to the housing for the sensor element surrounding it. During this welding step, through the slender tubes, mechanical stresses in the very sensor element that is constructed substantially of ceramic material can be very easily introduced, due to the heat generated by the welding flame used in the welding procedure. The mechanical stresses generated in this welding process to the lid or the housing of the sensor element depend on a lot of factors, the diameters of the tubes, the welding velocity, the cooling method, the degree in which it is possible to simultaneously make the two welds, etc. The final result is that each sensor mounted in a housing will obtain individual characteristics which differ rather much from each other. Thereby the problem of the accuracy of the sensor is transferred to a final calibration of the sensor itself, where these various imperfections must be acted on or treated individually and be compensated in different complicated ways.
An alternative to an attachment by means of two slender tubes is by means of one single centrally mounted tube, to which a housing portion or annular portion associated with or connected to the sensor element of a ceramic material is welded.
For an attachment by means of two slender tubes the whole sensor element is exposed to the pressure to be measured what is an advantage in particular when measuring pressures above the atmospherical pressure or above the ambient pressure. All the ceramic parts are then loaded or stressed by the same compression forces what the incorporated ceramic materials can stand very well. This advantage for measurements above the atmospherical pressure is not as pronounced for measurements of lower pressures or for measurements of a vacuum.
For an attachment comprising a single, centrally welded tube, through which the fluid enters the pressure of which is to be measured and which acts on only one surface of a sensor element, worse characteristics are obtained in the corresponding way for measurements of pressures above the ambient pressure. Since only one surface of the pressure element is loaded, the element can be "blown up" approximately in the same way as a balloon. This effect does naturally not exist for pressure sensors intended for measurements of low pressures and in that case this attachment method works satisfactorily. Such an attachment by means of a central tube welded to a housing portion in the shape of a circular plate of metal having on one side an annular projection or bead at the circumference and having a central aperture, can at present stand an overpressure of about 10-15 bar. Above this pressure a risk of bursting exists which will be manifest around or at the joints between the ceramics parts of which the sensor element is constructed, or by the fact that alternatively the diaphragm in the sensor element breaks. The security factor against rupture will thereby be low. Owing to the required processing steps at high temperatures further a distribution of pressures is obtained for which a rupture occurs, what in addition results in an unacceptable insecurity in this constructional method.
From the European Patent Application EP-A2 0 549 229 a pressure transducer is previously known comprising a sensor element having a ceramics house 38 and a diaphragm 36 arranged thereon which is made of metal (Inconel). Between an exterior stable support ring 42 an intermediate ring 86 is provided, which is welded at its one edge surface to the stable support ring 42. The other edge surface is connected at a shoulder or step and by means of glass joint to the ceramics house 38, at an angular projection thereon. The intermediate ring 86 is made of metal ("Inconel") having a coefficient of thermal expansion adapted to the material in the housing 38 and has a narrow web between the surfaces where the ring 86 is attached to the exterior support ring 42 and to the ceramic; housing 38. This construction reduces the transfer of the mechanical stresses which can arise at temperature changes due to different thermal expansion coefficients of the support ring 42 and the intermediate ring 86. In order to further reduce the influence of these stresses the intermediate ring is slotted, see item 98 of FIG. 4. However, such a construction will make the mounting of the sensor element less definite, as considered totally, and reduces the precision of the finished pressure sensor. The attachment to an annular projection on the sensor element reduces the thermal transfer and the projection can also break rather easily. Further, the fluid the pressure of which is to be measured and which is present at the exterior surface of the diaphragm 36, will also act on the whole sensor element, i.e. also on the rear side of the ceramics housing 38, what can be a disadvantage in certain cases, such as for measurement of pressures of liquids, in the case where a cleaning of the measurement volume may be required.